The present invention relates to a method of automatically recognizing of ultrasonic response signals resulting from interaction of ultrasonic test signals transmitted into structural material for purposes of nondestructive inspection and flaw detection. The invention will find particular utility in the pulse testing of welding seans, but is applicable to other types of tests as well.
Generally speaking, ultrasonic, nondestructive tests are carried out in that test signals, usually, pulses are transmitted into a test object and a receiving transducer "listens" to a response. An evaluating scheme must be prepared to particularly identify ultrasonic response signals resulting from the interaction of a transmitted ultrasonic pulse with the structural material to be inspected and to discriminate such responses from noise and to identify and classify, if possible, specific types of response signals on the basis of established, usually empirically acquired criteria. This is true regardless whether the test is conducted by an individual or whether the test results and signals are evaluated automatically. The currently used test methods are carried out on the basis of extensive rules and guide lines which have been worked out for many known specific tests and types of tests. Standards have been established and adopted in a variety of ways; additional standards are being worked out.
One of the test parameters of importance is the so-called sensitivity as a criterium to distinguish noise from particular (but possibly rather low level) responses. The magnitude of the test sensitivity in any particular case is that quantity which determines the minimum signal amplitude to be recognized as information. This quantity by itself is not decisive as to the absolute flow detection capability or resolution of the particular test method and system as a whole.
The test sensitivity of test equipment is, for example, adjusted and determined by means of reference objects or by means of the so-called amplitude comparing method (see "Archiv fur das Eisenhuttenwesen", 11/1959, pgs. 693-703). Automatic test equipment meets the test sensitivity as per prescribed rules in that all signal peaks exceeding a set threshold level and occurring within a particular period of time (in which response signals or a particular response signal, if occurring at all, will, in fact, occur) will be recorded and/or inspected and analyzed further. Signal peaks below that level are regarded as noise to be rejected, the reason being that very small defects may produce, for example, very weak echos equal in amplitude to noise so that a reasonable distinction is no longer possible.
The absolute test sensitivity is further limited, for example, by unique properties of the test object, by the general conditions under which the test is conducted, and by the specific mode of coupling the test transducers to the test object. Furthermore, the noise of the electronic equipment driving and receiving the test transducers are factors which may effect the degree of discrimination between noise and information.
Noise is generally detected as a jittery oscillatory variation of the signal level as received with peaks extending up to a not too well defined amplitude level. This level must remain below the response threshold to which the system is adjusted, otherwise noise will be detected as information. However, if the level spread is too large, valuable information may be lost and the test will fail to yield detection results on smaller defects.
The response and threshold level is usually fixed in the case of automatic or semiautomatic testing. The noise level to be expected has been empirically ascertained, e.g., through prior test runs involving, e.g., a test object known to be free from defects. One can reasonably expect that such a threshold level remains valid for a longer period of time of testing. The tests are then conducted and the signals are evaluated by detecting signal amplitudes which exceed that response threshold.
Nonautomatic testing may well operate under utilization of signals below the noise peak level and the discrimination is left to the operator. Automatic equipment cannot be expected to distinguish between noise and information signal peaks of like amplitude, so that the response threshold for distinguishing noise from information and ultrasonic response signals must be set correspondingly high. On the other hand, the effective sensibility of the system may vary during a test, or during sequential tests involving the same or adjacent zones of the test object. That is to say that under comparable conditions, which should produce similar responses, the responses are, in fact, not similar because of changes in the system which changed its sensitivity. Amplitudes of signals being comparable to the adjusted threshold may register in one instance but not in the other on account of such variations.
Various ideas have been advanced to keep the sensitivity of the equipment constant. For example, a special test signal is transmitted into and through the test object along a well defined path, e.g., straight through or as V beam. The received signal is used for purposes of adjusting the system's sensitivity on subsequent tests. However, this method depends to a considerable extent on the local geometry of the test object which, on the other hand, should not have any bearing on the sensitivity. Also, either of a pair of transducers which cooperate for this special test, may undergo variations in coupling to the test object. It should be noted here that variations in the coupling between transducer(s) and test object(s) are one of the primary sources of variations in the systems sensitivity. Even if all other conditions, including error sources, remain similar, it is impossible to decide which of the two heads produces these variations, and, therefore, the variations in the system's sensitivity. Consequently, one does not know which of the two heads and which of the respective circuits have to be adjusted for compensation of the variations in sensitivity.
Another known method proceeds as follows. Two transducers are mounted in a common head so that their conditions of coupling to the test object are the same and any variation will influence both of them. One of these heads directs a control pulse straight down and receives a rear wall echo to be used as a reference. The circuit for the other transducer is then adjusted accordingly. However, if that other transducer transmits into a different direction, the sensitivity so adjusted is not really representative. Variations in coupling may be compensated but other variations in sensitivity based, e.g., on normal texture changes, different grain orientations, etc., may be missed. This type of sensitivity control is not very reliable and can actually cause a deterioration in the performance of the test equipment. Utilization of separate pulses for purposes of the system's adjustment, has the added disadvantage of introducing delays.
It can thus be seen that there is a need for greater consistency in the sensitivity of an ultrasonic test system particularly as far as recognizing ultrasonic response signals and classifying them in a manner that permits consistent detection of flaws and defects is concerned.